Polyester resins such as polyethylene terephthalate (hereinafter PET) and the like are well known thermoplastic materials commonly used in the production of monofilaments. These monofilaments are frequently woven into support belts or fabrics for transporting and dewatering paper sheets produced by paper-making machines. While in use, these fabrics are subject to demanding conditions which mechanically wear and abrade the monofilaments from which the fabrics are made. As a result, paper-making fabrics which are comprised of polyester monofilaments generally may require replacement within about 30 to 60 days on wear prone forming positions. Nylon monofilaments are often used in combination with polyester monofilaments on high wear positions. The use of nylon may cause some problems in this type of usage due to its high moisture absorption. Accordingly, polyester monofilaments having an increased resistance to abrasion have long been sought by those in the paper-making industry.
It has long been known in the art to blend certain fluoropolymers with various thermoplastic resins to achieve a number of desired results. For example, Busse et al. U.S. Pat. No. 3,005,795 teach the blending of polytetrafluoroethylene (hereinafter PTFE) in powder form to various thermoplastic polymers such as methacrylate polymers, styrene polymers, and polycarbonates. Schmitt et al. U.S. Pat. No. 3,294,871 teaches the blending of PTFE in latex form to various thermoplastic polymers including those mentioned hereinabove. However, in both of these patents, the blends included finely divided microfibrous particles of PTFE which are not suitable for producing monofilaments as discussed hereinbelow.
At least two patents have blended PTFE with a polyester resin. Notably, Lucas U.S. Pat. No. 3,723,373 teaches the addition of a PTFE emulsion to polyethylene terephthalate (PET) to achieve a material which has greater elongation and improved impact strength. The PTFE emulsion is merely PTFE in the form of a latex dispersion or emulsion with water, mineral oil, benzene or the like. Accordingly, the PTFE emulsion also includes particles of about 0.1 micron to about 0.5 microns in size which comprise about 30 to 80 percent of the emulsion. The PTFE emulsion forms about 0.1 to 2.0 percent by weight of the blend, based upon the weight of the PET. Furthermore, Lucas indicates that this material can be extruded into sheet or stock shapes at a temperature of around 260.degree. C.
Similar to Lucas, Smith U.S. Pat. No. 4,191,678 relates to a fire retardant polymer blend comprising an aqueous colloidal dispersion of PTFE and a polyester resin. Again, however, the PTFE in the dispersion has an average particle size of about 0.2 microns. Smith also indicates that the blend may be subsequently extruded at about 240.degree. C.
The extrusion temperatures of these blends have been noted because it is well known that the melt temperature of PTFE is between about 335.degree. C. and about 343.degree. C. (635.degree.-650.degree. F.), and therefore, when PTFE and the polyester resin are extruded under standard operating conditions at temperatures below 320.degree. C., such as taught by at least one of the above-identified patents, it is clear that the PTFE in the blend must be in the form of solid particles and not in the form of a liquid melt. Importantly, such blends having PTFE in particle form have been found to produce monofilament which are insufficient for use in paper maker fabrics. The monofilaments are very difficult to extrude because the particles can easily clog or otherwise damage the extrusion equipment which is geared toward producing monofilaments from melted blends. Additionally, when monofilaments are produced from these blends, they have been found to be very rough and not suitable for use in paper maker fabrics. Furthermore, and possibly even more importantly, the PTFE retains its useful properties only up to about 287.degree. C. (550.degree. F.). Accordingly, by melting the PTFE at higher temperatures, all advantages gained by the inclusion of PTFE in these blends would be lost.
Thus, the need exists for a polyester monofilament having improved toughness and abrasion resistance which may be produced from a polymer blend of a polyester resin and a melt extrudable fluoropolymer under standard operating conditions.